When you compose a message in Mail, then tap "+" in the address field (to the right of To:), you will see all your contacts listed. If you want to only see contacts from a particular group, tap Groups on the top left, then tap the different groups so only the group you want to see is checked, then tap Done and you will only see contacts belonging to that group.
If you mean that you want to address email to all contacts in a particular group without selecting them individually, you need to use an app like Speed Names to do this.
Thanks for the tip. I have used apple products for 12-15 years, I haven't come across any quite so non intuitive or inconsistent with other apple devices! On my MacBook for years I have typed the name of the group, and mail puts all the addresses of that group in the address field.
I will look for the app you suggested! Thanks again!!!!